Silicone-acrylate impact modifier

Abstract

A silicone-acrylate impact modifier composition, wherein said impact modifier composition comprises structural units derived from: at least one silicone rubber monomer, a branched acrylate rubber monomer having the formula: wherein R<1 >is selected from hydrogen and C1-C8 linear and branched hydrocarbyl groups; and R<2 >is a branched C3-C16 hydrocarbyl group; a first graft link monomer, a polymerizable alkenyl-containing organic material, and and a second graft link monomer. The silicone-acrylate impact modifier compositions disclosed herein are useful for making molding compositions, which are useful for producing various articles, especially for outdoor applications.

Description

[0001] The disclosure relates generally to silicone-acrylate rubber compositions and their use as impact modifiers in resin molding compositions, particularly those comprising thermoplastic resins. Furthermore, the disclosure also relates to an emulsion polymerization method for making the silicone-acrylate impact modifiers. As used hereinafter, the expressions "silicone-acrylate rubber" and "silicone-acrylate rubber graft hybrid" mean an interpenetrating composite of silicone rubber and polyacrylate rubber, where the silicone rubber and polyacrylate rubber are entangled in an inseparable fashion at the molecular level.[0002] Butadiene-based impact modifiers, such as acrylonitrile-butadiene-s-tyrene (also called ABS) copolymers and methyl methacrylate-butadiene-styr-ene (also called MBS) copolymers have been previously used to improve the impact performance of thermoplastic materials. However, due to the presence of unsaturation, these butadiene-based copolymers respond poorly to we...

AI Technical Summary

Benefits of technology

[0030] The silicone-acrylate impact modifier compositions disclosed herein can be prepared by emulsion polymerization techniques. In the first step of the technique, at least one silicone rubber monomer is reacted with at least one first graft link monomer at a temperature from about 30.degree. C. to about 110.degree. C., and preferably from about 75.degree. C. to about 95.degree. C., to form a silicone rubber latex. An effective amount of a surfactant can be used initially in the reactor as part of the agitated aqueous mixture, or it can be introduced with the silicone rubber monomers. Surfactants that can be used include acid catalyst-surfactants, for example, sulfonic acids, such as alkyl-, and alkaryl-arylsulfonic acids and mixtures of surface-active sulfonic acid salts with strong mineral acids. Dodecylbenzenesulfonic acid is a preferred surfactant. In one embodiment of the method, the addition of monomers can be carried out batch wise or semi-continuously, and in a drop wise manner, over a period of up to 24 hours. The types of silicone rubber monomers and the first graft link monomers that can be used have been described previously. In another embodiment of the method, cyclooctamethyltetrasiloxane (hereinafter sometimes referred to as "D.sub.4") and tetraethoxyorthosilicate (sometimes also referred to as a silicone cross linking monomer) are reacted with (gamma-methaacryloxyprop-yl)methyldimethoxysilane (hereinafter sometimes referred to as "MAPMDMS") as a first graft link monomer to form silicone rubber particles. MAPDMMS facilitates chemical linking of acrylate chains onto the siloxane network. TEOS serves to form a weak cross-link in the silicone rubber particles. The average size of the silicone rubber particle depends on the cross-linking density. A higher cross-linking density generally results in a lowered particle size of the silicone rubber particles. In one embodiment, the method affords silicone rubber having an average particle size from about 100 nanometers to about 2 microns.

[0031] In the second step of the technique, at least one branched acrylate rubber monomer of the structure (I) is polymerized with the silicone rubber particles obtained in the first step to provide a latex comprising an emulsion polymerized silicone-acrylate rubber hybrid. In one embodiment, a branched acrylate rubber monomer of formula (I), such as isooctyl acrylate is polymerized with the silicone rubber particles in presence of a cross linking monomer, such as allylmethacrylate to get silicone-acrylate hybrid latex particles. Allylmethacrylate performs the dual function of cross linking the acrylate chains as well as acting as a graft linker (via the allyl group) for the grafting reaction in the third stage as described later in the disclosure. In another embodiment, a mixture of acrylate rubber monomers comprising the branched acrylate rubber monomers of structure (I) and linear acrylate rubber monomers, such as butyl acrylate can also be employed. The addition of the acrylate monomers to the silicone rubber latex occurs before, or concurrently with addition of a polymerization catalyst. The polymerization catalyst can be any material known in the art to initiate free radical polymerization, such as an alkali metal persulfate; or organic soluble radical initiators, such as azobisisobutyronitrile, or an organic peroxide, such as benzoyl peroxide, dichlorobenzoyl peroxide cumene hydroperoxide, and tert-butyl perbenzoate, to polymerize the branched acrylate rubber monomer and effect silicone-acrylate rubber hybrid formation. When an alkali metal persulfate catalyst is used, it is preferred that the persulfate be added over time to keep the vinyl polymerization running. This technique also minimizes degradation of the persulfate under the acid conditions present during the polymerization of the silicone rubber monomers. The emulsion polymerized silicone-acrylate rubber hybrid comprises about 95 parts to about 5 parts by weight of silicone rubber, and about 5 parts to about 95 parts by weight of polyacrylate rubber, per 100 parts by weight of said silicone acrylate rubber hybrid.

[0036] The silicone-acrylate impact modifier compositions disclosed hereinabove are useful materials for preparing polymer molding compositions having improved properties, such as stiffness, low temperature ductility, weatherability, and chemical resistance. The impact modifier compositions also confer unexpectedly, a higher melt volume rate to polymer and molding compositions during the processing step. A higher melt volume rate generally translates to easier processing of the polymer or molding compositions, which can be a significant benefit commercially.

[0043] The molding compositions comprising different polymer resins and the silicone-acrylate impact modifier compositions disclosed herein posses superior properties, such as low temperature impact and ductility, as compared to the polymer resin compositions which comprises methyl methacrylate-butadiene-styrene block copolymer impact modifier.

[0044] The previously described embodiments of the present invention have many advantages, including the ability to prepare the silicone-acrylate impact modifier compositions, and new molding compositions having superior low temperature impact, ductility, and good weatherability.

Problems solved by technology

However, due to the presence of unsaturation, these butadiene-based copolymers respond poorly to weathering.

Impact modifiers based on acrylonitrile-styrene-acrylat-e (also called ASA) copolymers avoid the issues faced by the butadiene-based polymers, however, these materials only have room temperature ductility.

However, the low temperature impact and ductility performance, as measured for example, by the ductile-to-brittle transition temperature (hereinafter referred to as "DBTT") is in some cases not up to the desired mark.

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